1. Field of the Invention
The present invention generally relates to measuring devices and, more particularly, to a device for measuring an S-dimension of container finishes.
2. Description of the Related Art
In the manufacture of containers, certain dimensions are required to be within predetermined tolerance limits in order for the containers to function properly. In particular, the upper surface of the container, commonly known as the finish, must be maintained within certain manufacturing tolerances in order to provide adequate cap retention for a liquid tight seal to be formed between the container finish and the container cap.
Variations in the dimensions of containers, particularly plastic containers, may occur during molding or trimming operations due to many factors, including differences in the molds used to form the containers, shrinkage of the containers after molding, materials used, curing temperatures, and trimming operations.
In order to determine whether produced containers, such as bottles, are within predetermined dimensional tolerances, generally a sampling of the bottles being produced is measured to determine actual dimensions. This is especially true for the finish (i.e., neck portion) of the bottles. For bottles which will hold fluids, including consumer products such as detergents and bleaches, it is important that the S-dimension (i.e., start of thread dimension) is within predetermined dimensional tolerances so that a bottle cap will be retained properly on the finish and leaks will be prevented. The S-dimension is defined as the distance between the top of the sealing surface and the top of the thread or the uppermost location where a thread can extend as it is extended around the finish. The S-dimension is measured from the minor diameter of the thread or the base of the thread where the thread adjoins the outer wall of the finish.
The S-dimension of a container finish can be determined by manual measurement with a caliper. However, such measurements will include inaccuracies which vary depending on the expertise of a particular user. The inaccuracies occur due to the manual placement of the caliper, variations in the manual force applied to the caliper, and the caliper blades cutting into the soft material of the bottle finish, such as when the bottle is made of a blown thermoplastic. If the calipers are tilted just a slight amount, the reading will fluctuate. Therefore, the measurements are very difficult to repeat.
As an alternative to manual measurement with a caliper, container finishes may also be measured by an optical comparator. The optical comparator takes an enlarged shadow-graph of the bottle finish to provide a highly accurate measurement of the finish diameter. However, optical comparators are quite expensive and are generally not available at the location where the bottles are made. Therefore, when using an optical comparator, bottles often must be shipped to a laboratory for measurement, providing a very delayed determination of dimensional tolerances. As a result, a large number of reject bottles may be made before the error is corrected by adjustments to the blow molding and/or trimming processes.
In view of the above drawbacks of the known methods for measuring S-dimension of a container finish, it would be desirable to provide a measuring device for accurately measuring the S-dimension of a container finish rapidly and with minimal user error.
In one aspect of the present invention, a measuring device for measuring the thread start distance of a thread formed on an outer surface of a first container finish includes a base comprising a first surface for receiving a rim of the first container finish, a support member movably attached to the base and adjacent the first surface such that a side surface of the support member is in contact with the first container finish, a gauge attached to the base, and a gauge actuator operatively associated with the gauge. The gauge actuator measures a distance between the rim of the first container finish and a thread start point of the thread of the container finish as the gauge actuator moves along an upper base line of the thread. Specifically, the distance between the rim of the first container finish and a thread start point is the thread start distance of the first container finish. The measuring device further includes a counterweight attached to the base to balance the measuring device on the first container finish.
In another aspect of the present invention, a process for measuring the thread start distance of a thread formed on an outer surface of a container finish includes the steps of positioning a first surface of a measuring device on an upper end of the container finish, moving a support member of the measuring device in contact with a side wall of the container finish, contacting a gauge actuator with an upper base line of the thread wherein the gauge actuator is operatively connected to a gauge having a display; and moving the gauge actuator along the upper base line of the thread so as to measure a vertical distance between a thread start point and the rim of the container finish.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.